Hi,
Currently it's only possible to filter on lists. To distinquish public and non-public emails it's recommended to send them using seperate lists. This way things never get mixed up and you easily can make slections based on the lists.
As an alternative you can disable the display of emails in the frontend archvie.

The disadvantage is that you need to do this manually. It's only a matter of time until you forget to disable some email you don't want to show in the frontend.